Single crystal silicon spheres provide an ideal means of fabricating high efficiency photovoltaic devices. These devices are described in U.S. Pat. Nos. 4,021,323 and 5,028,546. The desired size of such spheres is approximately 750 microns in diameter. Most recently technology is being developed to use single crystal silicon spheres as well as other semiconductor materials as substrates in integrated circuits. Current applications envision the use of 1000 micron spheres, but ultimately a wide range of sizes to accommodate different circuitry is likely. In addition, it is likely that such applications will utilize spheres of other semiconductor materials, including germanium, gallium arsenide, and CuInGaSe.sub.2.
Silicon, unlike conventional materials,is difficult to form into spheres by such techniques as gas atomization, rotary atomization or shotting. During solidification a volume expansion occurs that causes spherical liquid silicon droplets to distort into highly irregular particulate. The irregular particulate that results requires extensive and expensive subsequent processing as described in U.S. Pat. No. 4,430,150 to convert the particles to a usable form. As a result other processes as described in U.S. Pat. No. 4,637,855 for spheroidizing individual grains of silicon and U.S. Pat. Nos. 5,431,127 and 5,614,020 for fusing and spheroidizing measured amounts of loose silicon powder have been developed. Some of these patents also describe how to heat silicon particulate to melt it and then cool the silicon from the molten state such that single crystal particulate is formed.
All of these processes require multiple manufacturing steps in order to produce single crystal silicon. It is well established in materials manufacturing that the more discrete steps there are in a manufacturing process the more costly the overall process. Thus all of the above manufacturing routes are inherently expensive as they first require particulate formation followed by additional special processes to convert the particulate to single crystals.
Kirby in U.S. Pat. No. 4,322,379 describes a unique process for producing near spherical, or tear drop shaped silicon particles that are almost single crystals, "usually having less than 5 crystallites in an 0.01 inch body" (250 microns) directly from the melt in a one step process.